1. Field of the Invention
This invention relates to a moving-body communication device which is installed in a moving body and transmits a response signal for answering an interrogation signal which is received in a communication area of an interrogator.
2. Prior Arts
An automatic toll-collection system in which the toll charges are settled without stopping vehicles at a tollgate has been considered. In a conventional automatic toll-collection system, when a vehicle passes through a predetermined communication area of an interrogator, various kinds of data are exchanged between the interrogator and a vehicle-mounted device through radio communication to automatically execute toll charging on the traveling vehicle. As a result, the occurrence of traffic congestion at the tollgate can be suppressed, and rationalization of the toll collection can be promoted.
In such an automatic toll-collection system, it is necessary to dispose an antenna of a road-side communication device serving as an interrogator at an entrance or an exit of a toll road. Also, it is necessary to provide a vehicle-mounted device for radio communication on each of the vehicles which travel the toll road. Further, in a communication processing to execute toll charging, it is necessary to reliably complete toll collection in a short time during which the vehicle passes through the communication area of the antenna.
In general, radio communication receives adverse influences from a wind shield and a wiper of a vehicle, the weather such as rain, snow, thunder, a device outputting radio waves of a micro wave frequency band (for example, an automatic speed regulation device), a radio communication device outputting illegal radio waves and the like. Therefore, a possibility that a transmission/reception error occurs in radio communication is higher than that in wire communication.
When the transmission/reception error has occurred, for example, when a vehicle-mounted device does not receive a transmission signal from a road-side communication device, which is to be received by the vehicle-mounted device, the vehicle-mounted device transmits a retransmission request signal to the road-side communication device so that the road-side communication device outputs the transmission signal again to maintain accurate communication as much as possible. In this case, as shown in FIGS. 14 and 15, signals are exchanged between the vehicle-mounted device and the road-side communication device.
Referring to FIGS. 14 and 15, first, a communication processing between the vehicle-mounted device and the road-side communication device is explained and then the problems of the processing shown in FIGS. 14 and 15 are explained.
The road-side communication device 1 outputs a signal so that each of vehicle-mounted devices 2 specifies a communication frame to execute communication without generating crosstalk even when plural vehicle-mounted devices 2 are in a communication area of the road-side communication device 1. That is, the road-side communication device 1 transmits the signal termed a frame control message (FCM) in advance, thereby specifying, for each of the vehicle-mounted devices 2, a time slot through which signals are transmitted therebetween at a first unit communication. Further, the FCM signal also provides up-link designation or down-link designation at a second and later unit communications.
When the road-side communication device 1 transmits data to the vehicle-mounted device 2, that data is transmitted as a transmission signal during a time slot that is specified by the FCM signal. When the vehicle-mounted device 2 receives the transmission data signal at timing of the time slot specified as described above, the vehicle-mounted device 2 returns an acknowledgment (ACK) signal representing the reception of the transmission data signal to the road-side communication device 1. Thereby, a single unit communication ends.
When the communication processing needs to be continuously carried out, the road-side communication device 1 transmits signals again. If a command instructing that the vehicle-mounted device 2 transmits a data signal to the road-side communication device 1 is provided as the FCM signal transmitted from the road-side communication device 1, the vehicle-mounted device 2 transmits the data signal during the specified time slot. When the road-side communication device 1 receives the data signal from the vehicle-mounted device 2, it transmits an ACK signal to the vehicle-mounted device 2. By repeating the unit communication in this manner, a sequence of the communication processing is carried out to complete toll charging.
In the case described above, after signals are exchanged between the road-side communication device 1 and the vehicle-mounted device 2 to establish a link sequence therebetween, a communication processing for transmitting data is carried out. In the process of the communication processing for transmitting data, if a radio signal reception error occurs, the following problems are caused.
For example, FIG. 14 shows a case in which, when the FCM signal of an up-link designation is transmitted from the road-side communication device 1, although the vehicle-mounted device 2 transmits a data signal to the road-side communication device 1 using a data response frame and the road-side communication device 1 returns the ACK signal to the vehicle-mounted device 2 in response thereto, the vehicle-mounted device 2 cannot receive the ACK signal even after the elapse of an expected time because of any problems. Because the vehicle-mounted device 2 does not receive the ACK signal, the vehicle-mounted device 2 determines that a signal reception error has occurred, i.e., the data signal transmitted using the data response frame does not reach the road-side communication device 1. Therefore, the vehicle-mounted device 2 waits until the road-side communication device 1 provides the FCM signal of the up-link designation again to retransmit the data signal to the road-side communication device 1.
On the other hand, because the road-side communication device 1 has transmitted the ACK signal in response to the data signal from the vehicle-mounted device 2, the road-side communication device 1 proceeds with the communication processing regarding the communication as being normally carried out. Therefore, the road-side communication device 1 transmits an FCM signal of a down-link designation for requesting data writing to proceed to a next sequence. It is to be noted that the reason that such a situation happens is in that the ACK signal itself is an acknowledgement signal, and there is no way for further ascertaining whether the vehicle-mounted device 2 can receive the ACK signal.
Because the FCM signal includes the down-link designation and it is not to be received, the vehicle-mounted device 2 cannot accept that FCM signal. Therefore, even though the road-side communication device 1 transmits the data writing request signal (the FCM signal of the down-link designation), the vehicle-mounted device 2 neither accepts it nor returns an ACK signal in response thereto. At this time, because the road-side communication device 1 cannot obtain a response signal responsive to the FCM signal of the down-link designation, it determines that a radio signal reception error has occurred at a vehicle-mounted device side and retransmits the FCM signal of the down-link designation (first retry).
After that, this retry operation is repeated. However, if the number of the retry operations reaches predetermined times, the road-side communication device 1 discontinues the communication processing regarding it as retry-out. At this time, because the vehicle-mounted device 2 does not receive the FCM signal of the up-link designation in a stand-by condition, the vehicle-mounted device 2 may fall in a state that it cannot escape from the stand-by condition if the worst happens. In this case, even if the vehicle-mounted device 2 releases the stand-by condition for receiving the FCM signal of the up-link designation when a time for the retry processing is out, it is impossible to avoid an inefficient communication processing. Further, such an inefficient communication processing limits communication times for the other vehicles which concurrently pass through the tollgate.
Also, FIG. 15 shows a case in which, when the vehicle-mounted device 2 receives a data signal transmitted using a data writing command frame following to the FCM signal of the down link designation from the road-side communication unit 1, although the vehicle-mounted device 2 returns the ACK signal to the road-side communication device 1 to terminate the communication processing, the road-side communication device 1 cannot receive that ACK signal. Because the road-side communication device 1 does not receive the ACK signal, it determines that a radio signal reception error has occurred at the vehicle-mounted device side. Therefore, the road-side 5 communication device 1 transmits the FCM signal of the down-link designation again to retransmits the data signal using the data writing command frame (first retry).
At this time, because the vehicle-mounted device 2 has completed the communication processing for toll charging by transmitting the ACK signal as the last signal of the communication processing, the vehicle-mounted device 2 is in a state for accepting a FCM signal of no link direction instruction from the road-side communication device 1 to start another communication processing. Therefore, even when the vehicle-mounted device 2 receives the FCM signal of the down-link designation which is to carry out the retry operation from the road-side communication device 1, the vehicle-mounted device 2 does not respond to this signal regarding the sequence of the communication processing as erroneous.
As a result, after the retry operation is repeated predetermined times in the road-side communication device 1, the road-side communication device 1 determines that the processing for toll charging is incomplete. That is, although the road-side communication device 1 only fails to receive the ACK signal as the last signal, and the communication processing for toll charging is substantially completed, the toll collection cannot be carried out.